it seems you have answered your own question re "why". all this manual fiddling you dont have to do when you use the system properly. ie it becomes the gateway for your entire network.
I posted my answer in case someone else finds this post in the future and wonders how I got it working. I did have to add both the IP and the MAC address to get the external share to work, then I added a link to it on the core's HDD. It didn't seem to show the files on it otherwise.
Ok I'll be honest here, the NAS is not behind LMCE simply because IMHO it isn't stable. I end up rebooting into Windows on the core to get a video to play or something and I need to get to the NAS when LMCE is off. I used to have it 80% working (zwave, remote, md's) and now it is not. I added a link to the NAS and now the MD's crash when I try to browse the videos (it did work for a couple videos before it was done running UpdateMedia). ZWave turns on/off all lights in the house when I play a video OR turns on/off the wrong ones, the Fiire and Gyration remotes I have no longer seem to work, and there are other misc problems that I haven't gotten to yet (sound works on one MD, but doesn't on another that is identical for example). I loved it when I had 710 and it worked, I may go back to that because it least it is the "devil I know".